The present invention relates generally to a winch and associated control circuitry used to pull or lift heavy loads, and relates particularly to an electronic current limiting device sensing a magnitude of current used by the winch and halting operation of the winch as a function of the magnitude of electric current drawn. While illustrated in the context of a winch, it will be understood that the present invention also relates to applications where an electric motor operates to move a load and where potential damage exists when the load force exceeds an acceptable load force threshold.
Winches support a wide range of applications and assume a variety of sizes and types. Winches typically have an upper load limit. A winch may be rated, for example, as a 10,000 pound winch indicating that load bearing components may withstand an applied load force to a maximum of 10,000 pounds.
Potential damage or actual damage exists when a winch is driven beyond its rating, i.e., placed under a load condition exceeding its rating. Without some form of shut-off mechanism, the weakest components of the winch potentially break or deteriorate. Unfortunately, destructive failure often presents the first indication that a load limit has been exceeded. In other words, often there is no warning that a damaging or destructive condition exists. Not only can winch damage or destruction be costly in repair services and shut down time, it also represents a safety hazard to those working with or in the vicinity of the winch. Even in cases where the components of the winch are designed to far exceed its load limit rating, damage may occur to supporting structures or to the item being pulled or lifted. The operator/user may simply want conservative assurance that the load limit will not be exceeded in any event. The subject matter of the present invention finds application in maintaining loads at or below a selected load force.
Sensing the load condition on a given winch may be accomplished by sensing the magnitude of current delivered to the winch. U.S. Pat. No. 4,873,474, issued Oct. 10, 1989 to Johnson and assigned in common to the assignee of the present invention, illustrates a winch with a shut-off load limiting device. Generally, the load limiting device of U.S. Pat. No. 4,873,474 senses a magnitude of current delivered to the winch and shuts off winch operation when current exceeds a given magnitude. The load limiting device of U.S. Pat. No. 4,873,474 allows adjustment in a threshold current used as a basis for triggering winch shut down.
U.S. Pat. No. 5,214,359, issued May 25, 1993 to Herndon, et al and assigned to the assignee of the present invention, also illustrates a winch having an electronic current limiter to shut off power to the winch motor when a load limit has occurred.
U.S. Pat. No. 5,648,887, issued Jul. 15, 1997 to Herndon, et al and assigned to the assignee of the present invention, also illustrates a winch system including an electric motor protected against an overload condition by use of a current sense device and a current limiting device. The device shown in U.S. Pat. No. 5,648,887 responds to specific state conditions of the device and operates to protect the electric motor against damage in reaction to the existing operating condition and state.
Each of the above-noted U.S. Pat. No. 4,873,474; 5,214,359; and 5,648,887 employ as a current sensing device a hall effect device and/or, a torid device. These current sensing devices are inherently slow devices and, when used as the primary current sensing device for purposes of detecting load conditions, introduce significant delay into the process of detecting a given load magnitude and reacting to a given load magnitude exceeding a selected threshold load. Accordingly, there exists need for a faster and more accurate mechanism for detecting load conditions and halting operation of an electric motor as a function of existing load conditions exceeding a selected threshold load magnitude.
Another major issue regarding use of hall effect transducers and torids is the interface required relative to the winch. To use a torid, for example, it must interface with an actual current carrying portion of the winch. Typically, the torid must encircle the bus bar, i.e., a major current carrying portion of the winch. This places the current sensing at a particular location within the winch control pack. This creates difficulties in integrating the electronic current limiting device with the winch. A second area of concern is adaptability. Prior electronic current limiting devices used a potentiometer to set a threshold load limit. Potentiometers are vulnerable to damage and suffer from relatively less accuracy in setting and less accuracy in consistency from unit to unit. Also, potentiometers can vary setting in response to vibration, and therefore may drift from a previous setting. Furthermore, unsophisticated users gaining access to the potentiometer can modify the threshold setting and potentially establish damaging or hazardous operating conditions. Accordingly, a versatile yet less accessible method of setting the threshold is desirable. It would be desirable, therefore, to provide an electronic current limiter locking into an established threshold setting without variation in response to vibration.
Furthermore, potentiometers required that the package could not be sealed permanently. Because the package required access for resetting a threshold value, the package could only be sealed in a limited fashion. Accordingly, it would be desirable to provide a current limiting device having adjustability in threshold values, but not requiring that the device by opened to allow adjustment in threshold value. Accordingly, such a device could be hermetically sealed permanently for better protection against moisture and debris.
Accordingly, it would be desirable to provide a simplified interface relative to an existing winch system whereby a current limiting device may be integrated without extensive modification to the winch system while also providing a highly accurate, fast current limiting protective device.